Note that there are 2 images per excerpt: one is rather large "featured image" cropped; the other is the proper thumbnail.

I just upgraded from theme TwentyEleven to TwentyTwelve.
With the TwentyEleven theme, it worked fine (though it wasn't always clear which image would be used for the thumb).
I've spent hours, read many posts and tried manual excerpts and various plugins to no avail. I am currently using the "Auto Excerpt everywhere" plugin.

The only way i have gotten any excerpt thumbnails is to set a post feature image. And then I get the two images on the excerpt. It also (as expected) adds the unwanted cropped feature image at the top of the post (example: http://richardbecker.com/sculpture-projects/sg2012 ) .

Any help would be greatly appreciated.
I am not php, html or css literate, though I do have a child theme set up and can edit.